In an optical transmission system, for example, there is a case where an optical modulator that performs optical modulation with the DP-DQPSK (Dual Polarization Differential Quadrature Phase Shift Keying) method is used. In the DP-DQPSK method, a light beam input to the optical modulator is first split into two light beams, and an electric signal is superimposed on the two split light beams. The two light beams having the electric signal superimposed thereon are then combined with each other.
In order to superimpose an electric signal on two split light beams, a ferroelectric crystal such as lithium niobate (LiNbO3) is sometimes used. In a case of using a ferroelectric crystal, because an electric signal is superimposed on the light beams in a waveguide within the crystal, crystals with a predetermined size are arranged, and therefore there is a certain limit on downscaling of optical modulators. In this connection, in recent years, an optical modulator that uses a semiconductor chip has been studied in order to achieve downscaling and high efficiency of optical modulators.
Further, in order to combine two light beams having an electric signal superimposed thereon, a polarization coupling unit including a polarization rotating element and a polarization combining element is sometimes used. The polarization coupling unit rotates the polarization direction of one of two light beams that travel in parallel to each other with the polarization rotating element such as a waveplate, and combines two light beams of which the polarization directions are perpendicular to each other with the polarization combining element such as a PBC (Polarization Beam Combiner) prism with each other.
Specifically, a polarization beam splitter film is formed on the PBC prism. The polarization beam splitter film is a reflection film with polarization selectivity. The polarization beam splitter film transmits light with its polarization plane parallel to an incident surface of the film (P-polarized light), and reflects light with its polarization plane perpendicular to the incident surface (S-polarized light). As a polarized wave of one light beam transmits the waveplate, the waveplate rotates the polarization direction of the one light beam to make it perpendicular to the polarization direction of the other light beam. That is, the polarized wave of the one light beam is turned from P-polarized light to S-polarized light.
For example, as illustrated in FIG. 7, a light beam 31 (a solid line in FIG. 7) and a light beam 32 (a dotted line in FIG. 7) are incident on the polarization coupling unit as P-polarized light. The polarized wave of the light beam 31 passes through a waveplate 12, so that the waveplate 12 rotates the polarization direction of the light beam 31 to make it perpendicular to the polarization direction of the light beam 32. That is, the polarized wave of the light beam 31 is turned from P-polarized light to S-polarized light. A reflection film 14 and a polarization beam splitter film 15 reflect the polarized wave of the light beam 31 incident from the waveplate 12 on a PBC prism 11. The polarization beam splitter film 15 transmits the polarized wave of the light beam 32 incident on the PBC prism 11. The PBC prism 11 combines the light beam 31 reflected by the reflection film 14 and the polarization beam splitter film 15 and the light beam 32 transmitted through the polarization beam splitter film 15 with each other.
In the configuration of the polarization coupling unit described above, as illustrated in FIG. 7, it is a common procedure to bond the waveplate 12 to the PBC prism 11. For example, the waveplate 12 is fixed to the PBC prism 11 with a fixing agent such as an adhesive. In this case, the fixing agent applied onto a bonding surface between the waveplate 12 and the PBC prism 11 may spread out of the bonding surface to a surrounding region to form a region, which is referred to as “fillet 13”.
The fillet 13 formed of the fixing agent in the surrounding region of the bonding surface between the PBC prism 11 and the waveplate 12 blocks traveling of light beams. Therefore, in a case of combining two light beams with the polarization beam coupling unit, the incident positions of the two light beams are adjusted in such a manner that the light beams travel along paths that bypass the fillet 13. Specifically, the light beam 32 is made incident on the PBC prism 11 at a position away from the surrounding region of the bonding surface between the PBC prism 11 and the waveplate 12. By adjusting the incident position of the light beam 32 in this manner, traveling of the light beam 32 is not blocked by the fillet 13.
Patent Document 1: Japanese Laid-Open Patent Publication No. 05-133800
Patent Document 2: Japanese Laid-Open Patent Publication No. 2015-169796
However, in a case where two light beams are input to such a polarization coupling unit, there is a problem that it is not possible to reduce the distance between the two light beams to a certain value or less. For example, in the polarization coupling unit illustrated in FIG. 7, the light beam 32 is made incident on the PBC prism 11 at a position away from the surrounding region of the bonding surface between the PBC prism 11 and the waveplate 12, and therefore a gap with a certain size is provided between the light beam 31 and the light beam 32.
In order to solve the problem of the polarization coupling unit illustrated in FIG. 7, that is, in order to reduce the distance between the two light beams, there has been conventionally proposed a polarization coupling unit that further includes a base member for fixing a polarization rotating element and a polarization combining element. Specifically, as illustrated in FIG. 8, for example, a base member 23 has a main body and two arms extending from the main body, and a cutout portion is formed between the two arms. A waveplate 22 is bonded to end faces of the two arms of the base member 23. To one surface of the two arms and the main body of the base member 23, a PBC prism 21 is bonded at a position opposed to the waveplate 22 and the cutout portion of the base member 23. The base member 23 is a base member to which the PBC prism 21 and the waveplate 22 are bonded. For example, the base member 23 is formed of a glass material, the PBC prism 21 is formed of quartz glass, and the waveplate 22 is formed of a quartz crystal. In addition, a reflection film 24 and a polarization beam splitter film 25 are formed on the PBC prism 21.
On the polarization coupling unit illustrated in FIG. 8, similarly to the polarization coupling unit illustrated in FIG. 7, the light beams 31 and 32 are made incident as P-polarized light. As the polarized wave of the light beam 31 passes through the waveplate 22, the waveplate 22 rotates the polarization direction of the light beam 31 to make it perpendicular to the polarization direction of the light beam 32. That is, the polarized wave of the light beam 31 is turned from P-polarized light to S-polarized light. The reflection film 24 and the polarization beam splitter film 25 reflect the polarized wave of the light beam 31 incident on the PBC prism 21 from the waveplate 22. The polarization beam splitter film 25 transmits the polarized wave of the light beam 32 incident on the PBC prism 21 from the cutout portion of the base member 23. The PBC prism 21 combines the light beam 31 reflected by the reflection film 24 and the polarization beam splitter film 25 and the light beam 32 having transmitted through the polarization beam splitter film 25 with each other.
Because the waveplate 22 and the PBC prism 21 are bonded to the base member 23 in the polarization beam coupling unit illustrated in FIG. 8, the waveplate 22 and the PBC prism 21 are not directly bonded to each other. Because there is no portion that bonds the waveplate 22 and the PBC prism 21 to each other, there is no fillet formed of a fixing agent such as an adhesive. Therefore, even if the distance between two light beams is reduced to a certain value or less, traveling of the light beam 32 is not blocked by a fillet.
However, in the polarization coupling unit illustrated in FIG. 8, the base member 23 is provided as a new component in order to solve the problem of the polarization coupling unit illustrated in FIG. 7, and this results in increase of the number of components. As a result, in a case of applying the polarization coupling unit illustrated in FIG. 8 to a device such as an optical modulator, the base member 23 is manufactured in order to reduce the distance between two light beams having an electric signal superimposed thereon. This configuration makes it difficult to sufficiently downscale the optical modulator.